1. Field of the Invention
The present invention generally relates to apparatus for controlling light emitting devices, and more particularly to apparatus for driving light emitting diodes with different spectrums by a feedback control system to produce different stable colors.
2. Description of the Prior Art
For the advantages of less volume, less input power, longer life and lower cost, light-emitting diodes (LEDs) are replacing conventional lighting devices, and novel applications thereof are emerging. For example, various colors could be generated by independently controlling the illuminance (or intensity) of two (or more) LEDs with distinct spectrum (or color) and mixing the color optically.
The LED is composed of N-type semiconductor and P-type semiconductor. The resistance of the interface (or node) between the N-type semiconductor and P-type semiconductor is susceptible to ambient temperature, and subsequently, the illuminance of the LED is likely to be affected by the resistance change. Specifically, the varying ambient temperature may result in an over-heated and over-lighted LED with high output, or alternately may result in an under-lighted LED with insufficient output. For example, in the constant-voltage driving mode when the ambient temperature rises, the interface resistance decreases, causing high operation power and heat for the LED and thus disadvantageously shortens the life of the LED; on the other hand, when the ambient temperature falls, the increased interface resistance causes low operating power for the LED, which renders the LED useless for its insufficient illuminance. Alternatively, in the constant-current driving mode, when the ambient temperature rises, the decreased interface resistance causes low operating power of the LED, which renders the LED useless for insufficient illuminance; and when the ambient temperature falls, the increased interface resistance causes high operating power and heat of the LED, which disadvantageously shortens the life of the LED. Further, the LEDs with different spectrums are susceptible to the ambient temperature with different degrees. Accordingly, it is difficult to precisely arrive at a required color by mixing the different spectrums.
For the foregoing reasons, a need has arisen to propose apparatus for controlling the LEDs that is capable of reducing the temperature affect on the LEDs, protecting to lengthen the life of the LEDs, stabilizing the output illuminance of the LEDs, and precisely mixing the colors of the LEDs.